Before committing to a dissertation or thesis project, it is good to become aware of any similar projects that might be underway or recently completed. Finding completed dissertations is the subject of a helpful page in the University Libraries "How To" Subject Guides: Dissertations and Theses. But finding dissertations that have not been finished is more complicated. Some scholarly societies have web sites that list dissertations in progress. The information in these sites sometimes come from surveys of academic departments and sometimes simply voluntary submissions by students.
Some similar sites exist for other disciplines. Here are a few in fields related to art history:
More casual sites managed by individuals have come and gone. (There was once a good one for architectural history.) "H-Sport Theses & Dissertations in Progress" is currently listing many examples in the history of sports. Searching keywords in the Humanities Commons can often connect to personal profiles, which sometimes include recent dissertations or works in progress. Many academic departments post lists of dissertations in progress within the department.